1. Field of the Invention
This invention relates to valve actuation systems in internal combustion engines. More specifically, this invention relates to hydraulic actuation systems for intake and exhaust valves in diesel engines.
2. Description of Background Art
Most internal combustion engines have actuated valves that selectively open and close to provide combustion air and remove exhaust gases from each cylinder. Internal combustion engines usually convert chemical energy from a petroleum-based fuel such as gasoline or diesel into mechanical energy. Diesel engines usually compress air in the cylinder and then inject fuel into the cylinder for the compressed air to ignite. The ignited fuel generates rapidly expanding gases that actuate a piston. Each piston usually is connected to a crankshaft or similar device for converting the reciprocating motion of the piston into rotational motion. The rotational motion from the crankshaft may be used to propel a vehicle, operate a pump or an electrical generator, or perform other work. The vehicle may be a truck, an automobile, a boat, or the like.
Many diesel engines have intake and exhaust valves near a fuel injector on the top of each cylinder. Each intake and exhaust valve usually has a valve element disposed in a passageway formed by a cylinder head. The passageway connects to the cylinder through an opening in a valve seat formed by the cylinder head. In intake valves, intake air flows through the passageway into the cylinder. In exhaust valves, exhaust gases flow out of the cylinder into the passageway. The valve element usually has a stem connected to a head. A spring typically is disposed on the stem. The spring biases the head into a closed position against the valve seat.
Diesel engines usually have an actuating mechanism connected to the stem of each valve. The actuating mechanism selectively presses against the stem, overcomes the biasing force of the spring, and thus moves the head into an open position away from the valve seat. The actuating mechanism may be a push rod, a rocker arm, a cam on a camshaft, a hydraulically actuated drive pin, a combination thereof, or the like.
The gap or clearance between the stem and the actuating mechanism defines a valve lash. The valve lash may adversely affect engine performance if the valve lash is too small or too large. If the valve lash is too small at lower temperatures, the valve may not close fully and or may open unintentionally during engine operation at higher temperatures. If the valve lash is too large, the valve may have a delay in opening and may not open sufficiently during engine operation.
In most engines, the valve lash is adjusted mechanically or hydraulically. The valve lash may be adjusted mechanically when the engine is cold using a lash adjustment screw or like device with gauges and probes to determine the valve lash position. The valve lash may be adjusted hydraulically by a hydraulic lash adjuster that uses engine oil to change the valve lash during engine operation. The valve lash adjustments typically are made to compensate for estimated variations in one or more physical parameters such as thermal expansion, valve seat wear, manufacturing tolerances, and the like. The valve lash adjustments may include allowances when the estimated variations are not known or can vary. However, the estimated variations and allowances may not be the same as the actual physical parameters for a particular valve. The actual wear on a valve seat may be more or less than the estimated wear. The actual manufacturing tolerances may be more or less than the estimated manufacturing tolerances. The difference between the actual physical parameters and the estimated variations and allowances may provide a valve lash that adversely affects engine performance.